PROJECT SUMMARY Neonatal quality improvement efforts are rightly credited with improving patient safety outcomes yet infants in neonatal intensive care units (NICUs) continue to experience preventable harm. A primary mechanism responsible for improved outcomes is the high degree of process reliability with which routine care processes ? especially those of nurses ? must be completed. Studies from our team and others show that NICU nurses regularly miss essential care linked to neonatal safety outcomes and that a primary contributor to nurses? low care reliability is high workload. Nurse workload is often operationalized as objective measures of nursing resource intensity using patient-to-nurse staffing ratios and/or patient acuity metrics. However, objective measures do not capture important experiential aspects of nurse workload that likely influence reliability such as cognitive demands and time pressure. Our prior research found that NICU nurses? subjective workload was more strongly and consistently associated with care reliability than either staffing ratio or infant acuity, suggesting that direct effects of ratios on care reliability may be overestimated. Further, routine monitoring of subjective workload in NICUs may be an important new strategy for addressing variation in care reliability. Our prior research was conducted in a single level IV NICU with high nursing resources compared to benchmarking units, limiting generalizability. We propose to evaluate relationships between nurse workload and care reliability in a larger, more diverse sample of NICUs, nurses, and infants. Using an intensive longitudinal design, we will enroll up to 210 nurses in 5 NICUs to report on workload and care reliability for approximately 820 infants over the course of 1,120 shifts. We aim to: 1) evaluate differential effects of objective and subjective nurse workload on care reliability in NICUs; 2) examine relationships between shift-level situational factors and nurses? subjective workload ratings; and 3) evaluate the validity of aggregating nurses? subjective workload ratings within a shift to inform real-time measurement strategies. The proposed study directly responds to NICHD PA-18-790 to ?study the effects of physician and nurse workload on performance and patient safety? in neonatal environments. Specifically, we focus on relationships between workload and safety- related performance, with implications for outcomes that we will test in a subsequent study. This project will advance patient safety by broadening current understanding of nurse workload and its effects on care reliability beyond staffing ratios and infant acuity measures, determining modifiable factors influencing nurses? subjective workload, and informing measurement strategies for subjective workload monitoring in NICUs.